Traditional film-screen radiography has been used as a medical imaging diagnostic system for many years. X-rays are projected through a patient's body part to irradiate a cassette containing a scintillator screen that converts the X-rays to light to form a latent radiographic image on a sheet of film placed in direct contact with the screen. The film is then chemically or thermally processed to produce a visual radiographic image that can be used by a health care professional for diagnostic purposes. Problems with conventional film based systems include delay in obtaining a diagnostic image, the requirement for chemical or thermal processing, and difficulty in providing the radiographic image outside of the immediate medical facility. These and other problems with film-based solutions have motivated the development of digital radiographic imaging systems.
One aspect of conducting radiographic imaging is the importance of controlling the amount of X-ray exposure that the patient and the detector receive since excessive exposure to this ionizing radiation can be harmful to the patient and would create undesirable density levels on film. Automatic exposure control (AEC) apparatus have been developed with this purpose in mind and are available with a broad range of film-based systems, including mammography systems. Automatic exposure controls (AECs) monitor exposure with detectors placed in the radiation path and sense this radiation level to automatically terminate the X-ray emission at the appropriate time. The AEC detector is typically placed in front of the imaging cassette when using higher energies as are typically used in general radiography. In the case of mammography, in which lower X-ray energy levels are used to improve subject contrast, the AEC detector is preferably placed behind the imaging cassette, in order to reduce the possibility of creating undesirable interference with the radiographic image caused by absorption of the X-rays by the AEC detector.
Recently, computed radiography (CR) digital systems have been developed for mammography that utilize reusable storage phosphor plates that are scanned to produce a digital radiographic image. The storage phosphor plate is typically contained in a cassette that can be the same size as film cassettes and the screen/cassette combination has radiographic attenuation properties that make it compatible with existing X-ray exposure and AEC systems. CR systems have been well received in the market since they provide many of the benefits of digital imaging while utilizing existing X-ray exposure systems thus minimizing the cost of converting to digital imaging. However, among other problems, the CR systems still result in a delay in obtaining a diagnostic image due to the necessity of removing the CR cassette from its position within the imaging apparatus and scanning the exposed CR plate.
Digital radiographic mammography is achieving growing acceptance as an alternative to film-screen and CR radiography systems. With digital radiography (DR), the radiation image exposures that have been captured on radiation sensitive layers are converted, pixel-by-pixel, to digital image data that is stored and subsequently displayed on electronic display devices. One of the driving forces in the success of digital radiography is the ability to rapidly visualize and communicate a radiographic image via networks to a remote location for analysis and diagnosis by radiologists, without chemical or thermal processing cost and delay and without delays in transmittal of hard copy processed radiographic films by courier or through the mail. Further, increased detective quantum efficiency (DQE) of DR detectors enables improved image quality at lower patient radiation dosage.
DR detectors can either be direct or indirect conversion devices. Direct detectors use a material such as selenium in contact with a TFT array for conversion of X-ray photons to electronic charge signals that are subsequently converted to a digital representation of the image. Indirect detectors use a scintillator screen for conversion of X-rays to visible light that is then detected via contact with an amorphous silicon photodiode and TFT array. Both types of DR detector have been shown to produce diagnostic quality images.
Today's solid-state, ionizing radiation-based image detectors (hereafter DR detector) used in projection digital radiography are relatively large and expensive. These detectors typically include the following major components: a protective housing; the X-ray converter material; a glass substrate with amorphous silicon circuitry that captures and selectively provides image signals on a pixel basis; high density interconnect circuits to receive readout commands and to transfer the image signals to conversion electronics; readout ASICs to amplify the signal charge and multiplex the signals for analog-to-digital conversion; and additional electronics to control the panel operation and transfer the digital image data to a host computer. There are a number of design constraints with these devices. For example, the low signal levels in these systems require that the physical distance from the detection panel to the readout electronics remain as short as possible to achieve acceptable signal-to-noise ratios, thus driving the detector assembly to contain a significant portion of its electronics components within the detector assembly enclosure. Many of the electronic components themselves require protection from the imaging X-rays, typically in the form of lead shielding, to reduce the risk of damage or malfunction.
Cost remains a significant problem. Complete mammography DR systems using this type of detector require substantial capital investment, as the system typically includes proprietary hardware such as the DR detector, operator interface, processing computer, X-ray generator, X-ray source, and patient positioner. As a result, DR systems are very expensive and the current market is small given the high cost of investment. Present DR detector based systems are not compatible with film-based systems, thus do not provide a digital imaging solution for the large installed base of X-ray imaging systems that presently support film or CR cassettes. It is therefore desirable to provide a DR detector based imaging system that is backward compatible with the large installed base of mammography X-ray imaging systems.
In an effort to ensure compatibility with X-ray exposure equipment, the dimensions of medical radiographic cassettes/screens/films are specified under International Standard ISO 4090:2001(E), entitled “Photography—Medical Radiographic cassettes/screens/films and hard-copy imaging films—Dimensions and specifications.” This specification encompasses both conventional film and CR phosphor screens, with nominal imaging areas up to 18 cm×24 cm and 24 cm×30 cm (metric origin) for mammography. Standard cassette dimensions are specified as part of this ISO standard, including the maximum cassette thickness of roughly 16.0 mm. The “free field for radiation detector” section of this standard specifies the area and X-ray transmission characteristics of the cassette/screen/film to ensure compatibility with conventional AEC systems.
There have been numerous types of X-ray equipment and configurations designed and used for specific radiographic procedures. For example, these systems include wall-stand, floor-mount, chest, or table units; designed for supine, upright, or other patient orientations. Particular systems have been designed to enable efficient operation of mammographic screening procedures. Major manufacturers of traditional X-ray equipment include, for example, Siemens, Philips, and General Electric. It has been estimated that worldwide installations of traditional mammography X-ray equipment exceed over 30,000 units. In order to serve owners and users of this sizable installation base and to provide them with the advantages of DR imaging technology, it would be advantageous to provide a retrofit DR detector that allows a relatively seamless transition from film-based imaging to digital imaging.
With this general goal in mind, there have been a number of proposed solutions to the problem of adapting DR imaging solutions to existing film-based X-ray systems, including the following:
U.S. Pat. No. 5,844,961 (McEvoy et al.), discloses a filmless digital X-ray system that is designed to be compatible with standard X-ray cassette housing external dimensions, but does not provide compatibility with existing mammography AEC systems.
U.S. Pat. No. 6,592,257 (Heidsieck et al.), and U.S. Pat. No. 5,715,292 (Sayag et al.), disclose small area mammography spot imaging detectors that are used for diagnostic procedures such as needle biopsies and are compatible with film cassette holders but do not provide full field imaging capability or compatibility with AEC systems.
U.S. Pat. No. 6,800,870 (Sayag), discloses a CR screen and reading system that is compatible with film cassette based X-ray exposure systems but requires readout of a storage phosphor plate after exposure, thus delaying image availability.
U.S. Pat. No. 6,734,441 (Wendlandt), discloses features of a CR cassette design that is compatible with AEC systems but still requires readout of a storage phosphor plate after exposure, thus delaying image availability.
As solutions such as these show, the need for digital retrofit to film-based systems has been well recognized. However, proposed solutions for DR retrofit apparatus have not addressed the particularly challenging requirements of full-field mammography imaging. A digital retrofit device must fit within the existing form factor of a mammography film cassette, requiring compact packaging. At the same time, AEC compatibility is an important requirement for compatibility with existing radiology systems, and provides even further constraints on component packaging. Providing the required chest wall access distance is yet a further challenge. Thus, it can be appreciated that there is a need for a retrofit digital mammography detector that offers DR advantages and that is compatible with existing X-ray equipment.